Add tests for multiple gpx files in archives

[gravitystorm]

This was fixed in ee458b6 , but since it was a hotfix it didn't contain tests. We should add some tests to prevent the same problem from happening again in the future, and so I'm adding this ticket in order that we don't forget.

[tomhughes]

Yeah that was already on my to do list ;-)

It proved a bit more complicated than I would have liked because both the libarchive wrapper and the zlib reader are doing the file access directly from C and bypassing FakeFS so we have to partially work on the real filesystem